<!DOCTYPE html >
<html> 
  <head> 
    <title>标签属性</title>
  </head>
  <body> 
    <div class="box">this is a div element 
      <div class="true"></div>
    </div>
    <input class="username" type="text" name="username" id="username"/>
    <input type="checkbox" checked="checked"/>
    <p style="color:#999999; font-size:18px"></p>
    <h2 style="color:#666666;font-size:20px;"></h2>
    <div class="foo"> </div>
    <div class="foo bar"> </div>
    <div class="foo bar"></div>
    <div id="box1"> </div>
    <p>这是一段文字 湖南幼专</p>
    <ul> 
      <li>item0</li>
      <li>item1</li>
      <li>item2</li>
      <li>item3</li>
      <li>item4</li>
      <li>item5</li>
      <li>item6</li>
      <li>item7</li>
      <li>item8</li>
      <li>item9</li>
    </ul>
    <script>
      var txt = '中华人民共和国'
      if(txt){
          console.log(txt)
      }else{
          console.log('undefine')
      }
    </script>
  </body>
</html>